1. Field of the Invention
The present invention relates to a process for demetallizing primary products of the oxo synthesis with steam.
2. Discussion of Prior Art
In the known technical oxo processes, olefins are converted with carbon monoxide and hydrogen at elevated temperature and pressure in the presence of cobalt compounds. The products thereby formed ("primary products"), consisting mainly of primary aldehydes and alcohols, contain the cobalt used as catalyst dissolved in the form of cobalt carbonyl compounds. These cobalt carbonyls complicate the further processing of the primary products, e.g. their distillation and hydrogenation, to such an extent that, for economic reasons, they have to be removed in a separate operating stage ("de-cobaltization").
The economy of technical oxo processes depends decisively on carrying out the cobalt carbonyl decomposition in such a manner that the resulting cobalt or cobalt compounds can easily be separated from the organic reaction products and recycled to the hydroformylation stage without great effort and expense.
Of the many known processes for de-cobaltizing the primary oxo products, the use of steam is particularly characterized by its economy and the advantage that the higher condensation products contained in the crude oxo products are at the same time partially cracked. Depending on the reaction conditions employed, above all on the sulphur levels in the reaction system, various decomposition products are obtained from the cobalt carbonyl compounds in the steam treatment, namely cobalt metal or preferentially cobalt hydroxide or, especially in the presence of acids, cobalt salts of these acids.
It is known from French Pat. No. 1,018,055 to decompose the cobalt carbonyl compounds contained in the primary product of the oxy synthesis with steam, with the formation of finely divided cobalt metal. After cooling the mixture and removing the gases from the head of the reactor, the metal can be separated by mechanical separation devices from the organic phase containing the products of the oxo synthesis.
In carrying out this process it has been found that in most cases only part of the cobalt is precipitated as metal, the rest being found in dissolved form in the aqueous phase. Additionally, this known procedure does not insure that the cobalt metal is always obtained in an easily separable, e.g. filterable, state.
According to the teaching of U.S. Pat. No. 2,779,796 the soluble cobalt compounds can, by treating the crude product with steam, be separated to such an extent by decomposition or precipitation that no extensive contamination of the connected apparatus occurs if, during the treatment of the crude product with steam, all contact with a fixed heating surface whose temperature is higher than the boiling point of the water-product mixture is avoided. In this process the crude product, instead of being treated in a stirred boiler, can alternatively also be treated by mixing together the streams of steam and oxygen-containing crude product in a turbulent flow field, such as exists for example in a mixer tube. This process can, however, only be used in the case of crude oxo products having very low Co contents of 0.1% and less. Such small Co concentrations in the oxo synthesis require over-large reaction volumes and high hydroformylation temperatures, and are thus economically unsuitable for, in particular, low molecular weight olefins. Moreover, despite the low Co concentration only about 90% Co is removed. With the Co concentrations conventionally used in this technology, Co would be deposited in a metallically stable and permanent manner on the stirrers and mixer tubes with this process.
The similar mixing of crude oxo product with hot water in a tube is described in German Pat. No. 10 24 499. With this process, too, the degree of de-cobaltization is, however, unsatisfactory and results in serious obstructions throughout the entire system as well as in the post-treatment apparatus.
It is an object of this invention, therefore, to provide a process which avoids the aforementioned disadvantages and guarantees a complete separation of the cobalt in such a form as to permit its direct reuse in the hydroformylation.